Beware: There are spoilers in this post.

Pity the season finale of “Boardwalk Empire.” The penultimate episode was so good, the payoff — while still outstanding — felt like a bit of a letdown by comparison.

This was nevertheless a landmark season for the HBO mob drama, which blew up many of its key relationships at the end of season one and teased that out into a sensational game of cat and mouse — Boardwalkempire11_22which incorporated the deaths of several major characters — in season two. Mostly, the conflict centered on the mobster/political fixer Nucky Thompson (Steve Buscemi) being betrayed by a number of those close to him — including his right-hand man Jimmy (Michael Pitt) and brother Elias (Shea Whigham) — and doing all he could to avoid winding up in jail.

To say the series produced unexpected twists and turns would be a vast understatement, but none was more jolting than the back story revealed the previous week detailing Jimmy’s incestuous relationship with his showgirl mother, which has turned into a career highlight for Gretchen Mol.

It’s easy to quibble, admittedly, about some of the moments in the show, particularly a wedding scene in Sunday’s finale that hewed a little too closely to the baptism sequence at the end of “The Godfather.” The depth of the cast also caused some of the most intriguing characters — including those played by Michael K. Williams and Michael Stuhlbarg — to recede into the background for too much of the season (although Williams had some scintillating moments early on). The story surrounding Michael Shannon’s tormented fed — who wound up being discredited and fleeing — also never fully congealed.

Even so, “Boardwalk” upped its game by most measures, and — near the hour’s sobering, gorgeously shot finish — clearly defined just how ruthless Nucky could be. In doing so, showrunner Terence Winter — whose credits include “The Sopranos” — has established this series as a mob drama right up there with it, if there was any lingering doubt.

“I am not seeking forgiveness,” Nucky said, before committing a cold-blooded execution.

The “Boardwalk” gang can rest easy. When you produce a show this good, no forgiveness is required.